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digitalmars.D.bugs - [Issue 19743] New: Foreach loops desugar into code that does not


          Issue ID: 19743
           Summary: Foreach loops desugar into code that does not work
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody puremagic.com
          Reporter: monkeyworks12 hotmail.com

I have the following code:

Needle* find(Haystack, Needle)(return scope Haystack haystack, scope Needle
needle)  safe
    foreach (ref val; haystack)
        if (val == needle)
            return &val; //Error: scope variable __r2 may not be returned

    return null;

void main()  safe
    int[9] haystack = [0, 10, 5, 6, 2, 3, 9, 4, 5];
    int* foundVal = haystack.find(3);

From my understanding of `return scope` parameters, this should compile.
However, the foreach loop desugars into the following: { int[] __r2 = haystack[]; ulong __key3 = 0LU; for (; __key3 < __r2.length; __key3 += 1LU) { ref int val = __r2[__key3]; if (val == needle) return &val; } } The slicing of the static array obscures the knowledge that `haystack` is `return scope` from the compiler. Furthermore, as `__r2` is inferred as `scope`, the compiler thinks that I am escaping a reference to a local `scope` variable, and does not allow this code to compile. Unfortunately, due to https://issues.dlang.org/show_bug.cgi?id=19742, writing a manually-desugared version does not quite work: for (auto i = 0; i < haystack.length; i++) if (haystack[i] == needle) return &haystack[i]; //Error: returning &haystack[cast(ulong)i] escapes a reference to parameter haystack, perhaps annotate with return --
Mar 15 2019